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Golden Eagles: no tilting at windmills

Todd Katzner and several other scientists were puzzled by a vexing problem. They knew that the wind turbines at Altamont Pass Wind Resource Area (APWRA) were killing large numbers of Golden Eagles that flew into their spinning blades. Yet the population of Golden Eagles in the area had stayed relatively stable over the years despite this unnatural source of mortality. The researchers considered two possibilities. First, this population of eagles may have had unusually high birth rates or unusually low death rates from other sources to compensate for the high windmill-induced mortality. Alternatively, immigrant Golden Eagles might be replacing those killed by turbines.

Altamont Pass Wind Farm, California. Credit: Todd Katzner.

This question has important implications for conservation biologists. If immigrant Golden Eagles are replacing those killed by windmills at APWRA, then the apparent stability of the local Golden Eagle population may be at the expense of other populations that are providing APWRA with these immigrants. So, even though APWRA’s windmills are not directly causing local eagle populations to decline, windmills at APWRA (and other windmill sites) may be indirectly leading to a decline in other populations. So Katzner and his colleagues did genetic and molecular analyses of tissues remaining from these killed eagles to learn as much as they could about these eagles and where they came from.

Golden Eagle in flight. Credit: Michael J. Lanzone.

The researchers used tissue samples from 67 eagles that were killed at APWRA between 2012-2014. They subjected these tissues to a variety of genetic tests to determine the sex and age of each individual, and to evaluate the genetic differences between individuals killed by the windmills.

In addition, Katzner and his colleagues used stable isotope analysis to evaluate whether the killed individuals were local birds, or immigrants from afar. For this analysis the stable isotope ratio is the ratio of a rare and nonradioactive isotope of hydrogen (2H) found in the sample (feathers of killed birds) in relation to the common isotope (1H). A feather’s stable isotope ratio is very tightly correlated to the stable isotope ratio of the water the bird drinks. The last important point is that different regions of the world have different characteristic stable isotope ratios in rainwater. So if you can determine the stable isotope ratio of a bird’s feather, you can compare it to the world stable isotope ratio map, and determine where the bird most likely spent the previous year (once birds molt, their new feathers assume the stable isotope ratio of their new location). This approach will underestimate the number of immigrants, because some distant locales have a similar stable isotope ratio as APWRA, and birds from those regions will be incorrectly scored as being local.

Map of May-August stable isotope ratios (of 2H in rainwater). Same colors represent similar stable isotope ratios, ranging from relatively high ratios (deep orange), to relatively low ratios (dark blue). I don’t discuss the meaning of the circles and triangles in this blog post.

Based on this analysis, more than 25% of the dead eagles were immigrants to the area, with some birds originating from more than 800 km away. The researchers point out that APWRA might be particularly attractive to eagles looking for a home because it provides two types of resources that are important to these birds – visually open feeding grounds with easily-located prey, and a consistent updraft to facilitate relatively effortless flight.

Probability that an eagle killed at APWRA was local. If the probability was less than 0.5 the researchers scored it as immigrant; if greater than 0.5 the researchers scored it as local.

About half of the immigrants that could be sexed were juveniles or subadults. The researchers argue that the apparent stability of the population in the APWRA region is achieved by young immigrants replacing those birds that are killed by windmills.

Percentage of local vs. immigrant (nonlocal) Golden Eagles by age.

Katzner and his colleagues are concerned that APWRA functions as an ecological sink that attracts eagles, primarily from nearby western states, to replace those killed by windmills. High death rates are particularly problematic to slow-growing populations, such as Golden Eagles, which usually lay only two eggs, with generally only one surviving chick per breeding season (the larger chick often kills its sibling). The researchers also point out that windmills also kill many other animals, including numerous bat species, which also have slow-growing populations. They encourage the renewable-energy industry to develop technology that will reduce windmill-induced death. Such efforts are already underway, and there is preliminary evidence that newer generation turbines are reducing Golden Eagle mortality rates.